The present invention relates to percussive drilling equipment in which a hammer piston is impacted against a drill bit under the urging of pressurized air.
In percussion drills, such as that disclosed in Schindler U.S. Pat. No. 3,503,459 for example, a drill bit mounted in the lower end of a casing is rotated and longitudinally impacted in order to cut through hard earth formations such as rock. The longitudinal impacts are transmitted from a reciprocable hammer piston driven by compressed air. The compressed air, which is supplied by a compressor, is applied alternately to front and rear ends of the piston to produce reciprocation of the latter. During forward movement, the piston strikes the drill bit to promote cutting effectiveness. Extending centrally through the piston is an exhaust conduit which leads to the front of the drill bit. Surplus air, i.e., air over and above than the amount necessary to actuate the piston, is permitted to flow forwardly through the exhaust conduit and outwardly from the front of the bit in order to cool and clean cutting elements and to flush cuttings from the bore hole.
In practice, an air compressor of fixed output pressure is used to drive different percussion drills having different air requirements. Therefore, the amount of surplus air may vary from one drill use to another, necessitating that the cross-sectional size of the exhaust conduit be changed to ensure that no more than the available surplus air is delivered to the front of the bit.
Heretofore, such changing of the conduit size has been achieved by means of choke plugs removably disposed in the exhaust conduit. A selection of choke plugs with different cross-sectional through-bores is kept on hand at the drilling site so that the proper size choke plug can be installed in order to adapt the drill motor to the compressor output. Such a procedure of maintaining a set of choke plugs creates a problem because the choke plugs, due to their small size, are easily lost or misplaced Thus, it is not uncommon for a drilling operation to be subject to delays while suitable choke plugs ar being sought.
It is conventional to provide a check valve which prevents a back flow of subterranean fluid rearwardly through the drill string. In the above-mentioned Schindler patent the check valve is mounted in a valve guide which is sandwiched between a rear sub and a tubular member. The tubular member includes a radial shoulder which rests upon a radial ledge of the casing. A gasket is disposed between the valve guide and the tubular member to compensate for dimensional tolerances between those parts. During a drilling operation, the tubular member is subjected to considerable vibration which can result in breakage of the shoulder and/or ledge. A prior art arrangement disclosed in Ingersoll-Rand Brochure No. 4512-C, dated 1987, eliminates the tubular member and gasket and instead mounts the valve guide directly in central opening of the rear sub. The valve guide contains a radially outwardly projecting shoulder which rests upon a radial ledge on the rear sub. The arrangement is such that, during assembly of the parts, the valve guide must be inserted in a rear-to-front direction through the central opening in the rear sub. This results in a number of disadvantages. For example, the rear end of the central opening must be of sufficiently large diameter to admit passage of the valve guide and thus must be in the form of a female coupling (i.e., an inwardly threaded hole) as opposed to a male coupling (i.e., an outwardly threaded post). That is, a male coupling does not possess a sufficiently large cross-section to contain an internal opening large enough to afford passage of the valve guide. Consequently, in cases where a male coupling is needed, it is necessary to attach a separate adapter to the female coupling for providing a male coupling at the rear end of the rear sub.
Furthermore, since the central opening must be at least as wide as the sealing head of the check valve, it is not possible for the valve seat to be defined by the inner surface of the rear sub which forms the central opening. Rather, it is necessary to employ a separate seat-defining sleeve inserted into the rear sub.
It will be appreciated that the need to provide separate adapter and seat-defining components increases the cost and assembly effort associated with such a percussion drill.